The present invention relates generally to frequency spectrum measurement apparatus and more particularly to a non-contiguous sweeping frequency spectrum measurement apparatus.
In traditional frequency spectrum measurements, the input signal spectrum is measured by linearly sweeping or stepping through a desired frequency range, bandpass filtering the signal and measuring the signal power after the bandpass filter. The time it takes to sweep the entire frequency range is highly dependent on the bandwidth of the bandpass filter, the frequency span and the frequency step size. To ensure the accuracy of the spectrum measurement, the frequency step size cannot be greater than 3 dB of the bandpass filter bandwidth. The point index of a conventional sweep may be mathematically expressed as follows:
f1f2 . . . fm f(m+1)f(m+2) . . . f(m+m) . . . f(n*m+1) f(n*m+2) . . . f(n*m+m)
where f1f2 . . . f(n*m+m) are frequency steps. f1 is the start frequency and f(n*m+m) is the stop frequency. FIG. 1 illustrates the conventional sweep with the start frequency at 10 KHz, the stop frequency at 120 KHz and the frequency step size of 10 KHz. The frequencies are swept linearly from the start frequency of 10 KHz to the stop frequency of 120 KHz and the signal power at each frequency point is measured. For a given frequency span, a narrow bandpass filter provides a better spectrum resolution but the number of frequency steps to sweep across the frequency span increases. The spectrum measurement can also be performed using a wide bandpass filter with larger frequency step sizes resulting in lower spectral resolution.
What is needed is a frequency spectrum measurement apparatus that improves the perceived spectrum update rate while maintaining a high spectrum resolution.
Accordingly, a frequency spectrum measurement apparatus includes a synthesizer that generates a variable frequency output signal over a frequency range. The variable frequency output signal is mixed with an input signal in a mixer to produce an intermediate frequency signal. A controller generates a non-contiguous sequence of input values to the synthesizer for generating over the frequency range a non-contiguous frequency output signal from the synthesizer with each value offsetting a current frequency output signal from a previous frequency output signal. Means are provided for measuring the power of the intermediate frequency signal.
The controller includes a number generator for producing the non-contiguous sequence of input values to the synthesizer for generating the non-contiguous frequency output signal from the synthesizer having multiple frequency scan lines. The number generator produces at least a first and a second series of non-contiguous input values to the synthesizer for generating at least the first and second frequency scan lines. The first series of non-contiguous values produce a frequency scan line having a start frequency and a series of stepped frequency values corresponding to the first series of non-contiguous input values. The second series of non-contiguous input values produce a frequency scan line having a start frequency offset from the start frequency of the first scan line and a series of stepped frequency values offset from the stepped frequency values of the first scan line and corresponding to the second series of non-contiguous input values.
The frequency spectrum measurement apparatus further includes a bandpass filter that receives the intermediate frequency signal and generates a filtered intermediate frequency signal output that is amplified and coupled to an analog-to-digital converter. The analog-to-digital converter generates signal samples representative of the bandpass filtered intermediate frequency signal and the controller processes the signal samples for generating signal power values representative of the intermediate frequency signal. The average power of the input signal over the frequency range is displayed on a display device.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawings.